The preferred embodiment of the method and device concerns an electrophotographic printing device, in particular a printer or copier, in which at least one printing group is used that comprises at least two exposure units, at least two charge corona devices and at least two developer stations that are arranged along the circumference of a toner intermediate carrier. The preferred embodiment also concerns a method for printing.
In a conventional printing device, a latent image is applied on a toner intermediate carrier with the aid of an electrophotographic method, for example via exposure of a photoconductor or via magnetization of a magnetically sensitive layer. Toner accumulates on the latent image according to the image-shaped distribution of the electrical charges or of the magnetic field lines. The toner is then transferred to the transfer printing location on a carrier material, for example a paper web. The toner image is later affixed to the carrier material.
In modern printing technology, it is required that a single device prints the carrier material on both sides with high speed. This operating mode is generally designated as duplex printing. The operating mode “spot color printing” or “two-color printing” is also required, in that the carrier material is printed two-color on both sides. Moreover, the requirement exists of a full-color printing with the four process colors “yellow, magenta, cyan and black”. In order, for example, to realize the operating mode duplex printing, it is known to first print a continuous carrier web on one side, then to invert the carrier web and to print the second side with the same printing group. The transport path of the carrier web in the printer and in the necessary web inverter device is relatively long and requires a complicated and also error-prone transport device for the carrier material.
A further disadvantage is that the still un-fixed toner image on the one side of the carrier web can be smeared upon transport and thus the print quality is impaired or waste is produced. In order to prevent this, an intermediate fixing of the toner image can occur, whereby however the technical effort is increased. Due to the long transport path between the first transfer printing location for the first transfer printing of a toner image and the second transfer printing location, compliance with high precision registration of the carrier web can only be ensured with high technical effort.
From U.S. Pat. No. 6,141,523 (corresponding to the parallel WO 98/27466) by the same applicant, an electrophotographic printing device is known that comprises two identical printing groups whose transfer printing locations are arranged opposite one another. A continuous carrier web is directed through both printing groups and the front side and the backside of the carrier web are simultaneously printed. The printing groups can respectively contain a plurality of exposure units and a plurality of developer stations, such that a multicolor print can be realized on each side of the continuous carrier web. The aforementioned document is herewith incorporated by reference into the disclosure content of the present application.
From EP 0 629 931 A1 (applicant: XEIKON), an electrostatic printer is known in which a continuous carrier web is directed in the vertical direction between a plurality of toner image carriers. Each toner image carrier has a device generating a toner image. The toner is transferred from the toner image carrier to the carrier web at each transfer printing location. Via two-sided arrangement of toner image carriers along the vertically-running characteristic quantity, a duplex printing with various toner colors is possible.
From EP 0 433 444 B1 (applicant: Eastman Kodak Company), a printer is also known in which a plurality of developer stations are arranged along a photoconductor band as a toner image carrier. Each developer station can ink the charge image generated by an exposure station with toner of a predetermined color. The toner image generated on the photoconductor band is then transferred to the carrier material at a single transfer printing location.
From WO 98/18052 by the same applicant, a printing device with two similar printing groups is known to which individual pages are supplied via a common input. The individual pages are supplied to both printing groups in the printing device via guideways, such that both the front side and the back side of the individual pages are printed given a single pass of the individual pages through the printing device. For this, the transport paths for the individual pages are connected to two rings via connection paths. Each ring contains a printing group. The individual pages can be supplied to the one ring or the other ring and can also traverse both rings in succession, whereby an inversion of the individual pages can optionally also occur. In this document, inverter devices are also specified. This document is likewise included by reference in the disclosure contents of the present patent application.
From U.S. Pat. No. 5,526,107, a color printer is known which comprises a printing group with a printing roller. The printing roller prints an image on one page side at a first transfer printing location and, given further transport, this same printing roller prints an image on the other page side at a second transfer printing location opposite the first transfer printing location. The page is inverted during the transport.
A tandem development system is known from Xerox Disclosure Journal 22, Nr. 4, pg. 197-200. Successive toner images are superimposed on a band-shaped image carrier for printing of a color image. These superimposed toner images are transferred to a carrier material.
WO 98/27465, DE-A-39 06 630, DE-A-39 35 231 and WO 99/09459 are referenced as further prior art.